In an internal combustion engine such as a gasoline engine, air-fuel ratio feedback control for detecting the concentration of a specific gas in exhaust gas by a gas sensor mounted to an exhaust pipe and controlling a mixture ratio of fuel and air, based on the result of its detection is now being conducted to reduce CO, NOx and HC in the exhaust gas.
As the gas sensor for realizing such air-fuel ratio control, there has been known a full-region air-fuel ratio sensor (hereinafter also called simply “UEGO sensor”) wherein two cells (specifically, an oxygen pump cell and an oxygen concentration detection cell) equipped with electrodes on both surfaces of a solid electrolytic layer formed principally of zirconia are laminated with a hollow measurement chamber interposed therebetween, and exhaust gas is introduced into the measurement chamber via a diffusion resistance body to detect an oxygen concentration in the exhaust gas.
The electrodes of the respective cells that constitute the UEGO sensor are electrically connected to a controller for driving and controlling the sensor. The controller executes a process for allowing current to flow through the oxygen pump cell so that a voltage outputted from the oxygen concentration detection cell will be a pre-set value, thereby to control the oxygen concentration in the measurement chamber to a constant value and measuring an oxygen concentration (i.e., air-fuel ratio) in the exhaust gas from the value of the current flowing through the oxygen pump cell. Incidentally, the UEGO sensor is provided with one common wire (common harness) for commonly connecting the respective electrodes of the cells, facing the measurement chamber in order to supply a common reference potential to the respective cells that constitute the sensor.
Meanwhile, since no reference potential is supplied to the respective cells where the common wire is broken, the controller does not normally perform feedback control for making constant the oxygen concentration of the measurement chamber and hence the UEGO sensor is not able to measure the oxygen concentration.
Therefore, as an air-fuel ratio system having this type of gas sensor, there has been known one which diagnoses the presence or absence of a malfunction in common wire (refer to, for example, Patent Document 1).
The air-fuel ratio system disclosed in Patent Document 1 determines whether, in diagnosing a break in a common wire (COM terminal), potentials corresponding to terminal voltages at three connecting points (including connecting points to which the common wire is connected) for connecting respective sensor cells and a sensor control circuit fall within a predetermined range. Then, information about such a result of determination is superimposed on output signals at three output terminals (specifically, a VRPVS terminal, a VIP terminal, and a VVS terminal), and the break in the common wire is diagnosed based on the combination of the three output signals.    [Patent Document 1] Jpn. unexamined patent publication No. 2003-97432